Unlike electrical fuses or circuit breakers electrical shunts provide a conductive path between terminals in a circuit. Typically shunts provide such paths upon the occurrence of certain triggering events. Shunts are useful for instance in bypassing faulty battery cells that are connected serially in power systems. In such systems, overall system failure can result from a single-cell failure. Such failure occurs when a cell becomes non-conductive due to an open circuit. To bypass the faulty cell a shunt switch is connected in parallel to the terminals of the cell. This connection permits current to flow around the cell. In space-borne systems, highly reliable shunt mechanisms for bypassing faulty cells are desirable. This is because power supplies are critical for operating such systems and any repair of the supplies is virtually impossible.
Previously electromechanical switches were employed to bypass faulty cells. These switches were spring-loaded and triggered by fuses. In addition other types of switches were used for non-bypassing applications. These other switches employed various forms of triggering mechanisms. In particular certain other switches were triggered by so-called heat-to-recover (HTR) shape memory metal alloy elements. HTR elements change shape in response to thermal increase. Such thermal increase may be induced by current flow through the element.
Switches that employ HTR elements however function as electrical fuses, not shunts. U.S. Pat. No. 4,700,259 to Stokes discloses an electrical circuit breaking device. The device breaks a conductor tensively in response to applied current. Soviet Union Patent No. 672674 to Korobov discloses a fuse. The fuse causes an open circuit by changing shape when heated. Current is applied to heat the fuse. U.S. Pat. No. 4,473,859 to Stone et al. discloses a piezoelectric circuit breaker with a bender. The bender responds to voltage changes by deflecting. The deflection then creates an open circuit.
U.S. Pat. No. 4,490,975 to Yaeger et al. discloses a self-protecting and conditioning memory metal actuator. The actuator uses a shape-memory spring that contracts due to temperature change. Current flow causes the temperature to change. U.S. Pat. No. 4,524,343 to Morgan et al. discloses a self-regulated spring which operates an actuator. The actuator functions in a circuit-breaking application. U.S. Pat. No. 4,825,184 to Bloch et al. discloses a current-controlled inductor. The inductor coil is made of shape memory alloy.
U.S. Pat. No. 3,573,550 to Baker discloses an automatically-resetting transient protection device. The device employs diodes connected to a power supply. The diodes serve to clamp the supply voltage at a certain level.